Summary of the Invention
An improved spiral wound membrane element comprising layers of membrane material fluid tight sealed along 3 edges enclosing a permeate spacer creating at least one permeate envelope upon which a feed/retentate spacer layer is laid, along at least one membrane face, the entire multi layer arrangement being wound around a hollow central mandrel (which may be closed at one end) and to which the permeate envelope is in fluid communication through the fourth unsealed edge, creating a spiral wound module element is disclosed which is useful in separation processes wherein a pressure gradient is maintained across the membrane from a feed side to a permeate side, the improvement comprising using as the feed/retentate spacer at least one layer of a material having an open cross-sectional area of at least 30-70% and using as the permeate spacer at least three layers of material characterized in that the outer layers are a fine material having an open cross-sectional area of about 10 to 50% and a coarse layer having an open cross-sectional area of about 50 to 90% interposed between the aforesaid fine outer layers and wherein the fine outer layers are in interface contact with the membrane layers enclosing the permeate spacer.